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Old Oct 07, 2010, 11:06 AM
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Question
Current draw static Vs. in flight

What would be the difference in current draw from static to the current draw in flight @ WOT?
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Old Oct 07, 2010, 11:41 AM
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Ware, herts. U.K.
Joined Sep 2008
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Hard to say. Depends mostly on what pitch prop you are using and what revs it is running although there are other factors.
If you reckon in flight your actual current draw will just sneak under the limit of your ESC when it would go over the top static then I reckon you would be pushing your luck.
On the other hand if at WOT your amps are just inside the limit then you will probably be flying with enough of a margin - but provide plenty of cooling air.

Several people are posting the traces from EagleTree data loggers now. The trouble is I have not yet seen one with the current noted at WOT static and flying. I may just have missed it of course.
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Old Oct 07, 2010, 12:56 PM
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I'd like to say 'gee, this is easy because when the plane is moving the air is already going past the prop so static must be larger' but these things can be counter-intuitive.


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Old Oct 07, 2010, 01:50 PM
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The general consensus is that current draw in flight will be slightly lower.
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Old Oct 07, 2010, 02:37 PM
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I guess it also depends on the airframe drag. Im going to do a flight with my watt meter plugged in at the weekend and see what it says. Ill launch at 1/2 throttle, do one level WOT pass then land.
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Old Oct 07, 2010, 06:09 PM
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So your watt meter has memory and can show you the numbers after you land? Wish my Astro did that!
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Old Oct 07, 2010, 06:14 PM
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United States, AK, Fairbanks
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Quote:
I guess it also depends on the airframe drag
Airframe drag is one of the biggest factors. A plane with a lot of drag (Slow Stick) will fly much slower than the prop wants to (the pitchspeed), whereas a plane with very little drag (Sokol) will fly very close (within a few %) to the pitchspeed of the prop because there's very little to slow the plane down. Think of dropping a rock and a feather. The closer you get to pitchspeed, the more efficient your motor/prop becomes because they have to shove less air out of the way to make a revolution and their overall work load decreases. There are WAY better ways of explaining it

Now, our illustrative Slow Stick's motor might draw 18.7A on the bench in static testing and then drop to like 16.2A in the air at WOT. The Sokol, on the other hand, might say 33A static but only 28.5A in the air. These numbers are just plucked from the proverbial hat, of course, but they work.

Man, that'd be a fast little Sokol

***In the case of SOME very high-rpm, coarse-pitched setups (mostly small pylon racers and the like) the prop will actually be stalled (same idea as a wing, really, but spinning) in static testing and then load up in the air (a stalled wing is carrying very little of the plane's weight; a stalled prop is producing very little thrust. Same idea), so the in-flight draw will be higher than the static. You're not likely to have this issue but it's useful info.
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